Generally, a motor vehicle automatic transmission includes a number of gear elements coupling its input and output shafts, and a related number of torque establishing devices such as clutches and brakes which are selectively engageable to activate certain gear elements for establishing a desired speed ratio between the input and output shafts. The brake can be of the band type or disk type; engineering personnel in the automotive art refer to disc type brakes in transmissions as "clutches" or "reaction clutches".
The input shaft is connected to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is connected directly to the vehicle wheels. Shifting from one forward speed ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing or disengaging the clutch or brake (off-going) associated with the current speed ratio and applying or engaging the clutch or brake (on-coming) associated with the desired speed ratio.
Shifts performed in the above manner, using clutches and/or reaction clutches, are termed clutch-to-clutch shifts and require precise timing in order to achieve high quality shifting, especially in situations where the transmission is upshifted in response to at least partial release of the engine throttle by the operator of the vehicle. Shifts of this type are termed step-out or lift-foot upshifts, and may be accompanied by a sharp torque reversal at the input shaft of the transmission.
Under such circumstances, the sharp torque reversal is carried through to the transmission output shaft prior to the shift via the active torque establishing device, and the the driveline experiences a significant torque disturbance due to lash in the transmission gear elements. A second disturbance is often experienced when the output torque is reversed to a positive sense by the application of the on-coming torque establishing device during the impending upshift.
For this and other reasons, automatic transmissions commonly include freewheeling or one-way torque transmitting elements in series with a reaction clutch, which elements are capable of transmitting torque in one direction only. As soon as the applied torque reverses, the driving element overruns the driven element and no torque is transmitted therethrough. When the on-coming torque establishing device is applied the output torque increases from near zero with little or no torque disturbance. However, one-way devices do not come without expense, and it is highly desirable to limit their usage in transmissions.